The present invention relates to a power on detection circuit that detects the rise of an input voltage to a fixed value.
Generally, a power on reset circuit (also referred to as power on detection circuit) generates a reset signal for initializing an internal component of a semiconductor integrated circuit, such as a flip-flop, a latch, a counter, and a register when the power goes on.
Accordingly, there have been proposals for a voltage drop detection circuit that outputs a signal in accordance with the level of a power supply voltage. For example, National Semiconductor “LMS33460 3V Under Voltage Detector”, [online], [searched on Jan. 14, 2007], Internet <URL: http://www.national.com/ds/LM/LMS33460.pdf.> describes such a circuit. The voltage drop detection circuit includes a reference voltage power supply and a comparator to compare a divided voltage of the power supply voltage with a reference voltage and detect a voltage drop. The output of the comparator is provided to the gate terminal of an n-channel MOS transistor. This activates the MOS transistor and enables the detection of a voltage drop.
To accelerate the activation of a band gap reference circuit, a reference voltage monitor circuit that monitors a reference voltage has been proposed. For example, Japanese Laid-Open Patent Publication No. 2000-339962 (FIG. 1) describes such a circuit. The reference voltage monitor circuit described in this publication includes a band gap reference circuit. A reference voltage generation circuit generates a predetermined reference voltage from an external power supply voltage supplied from an external device. An internal voltage generation circuit generates a predetermined internal voltage from the reference voltage. A comparator compares a divided voltage of the power voltage with a reference voltage and generates a control signal for accelerating the activation of the band gap reference circuit until the potential of the reference voltage reaches a predetermined value.
Furthermore, to stabilize operation during the generation of a reset signal, a power on reset circuit that reduces current consumption after the reset signal is generated has been proposed. For example, Japanese Laid-Open Patent Publication No. 2001-292054 (FIG. 1) describes such a circuit. The power on reset circuit described in the publication includes a detection circuit and a flip-flop circuit. The detection circuit detects that the power level has risen to a predetermined value when the power goes on. The flip-flop circuit, which connects the inputs and outputs of two inverters, is operated in two stable states. The flip-flop circuit, which is initialized when the power goes on, inverts its signal when the detection level of the detection circuit reaches a predetermined value.
In the prior art, as described above, a circuit for detecting that the input voltage has become the proper voltage has been proposed. However, as described in National Semiconductor “LMS33460 3V Under Voltage Detector”, accurate measurements of the input voltage would increase current consumption. In the scheme described in Japanese Laid-Open Patent Publication No. 2000-339962, the comparator of the reference voltage monitor circuit constantly consumes current. This increases current consumption. Further, questions to the detection accuracy may arise due to the comparison of the divided voltage of the power supply voltage with the reference voltage.
In the scheme described in Japanese Laid-Open Patent Publication No. 2001-292054, the detection circuit, which includes two transistors, uses the threshold value of the transistors. Thus, the detection circuit has a relatively simple circuit and reduces power consumption. However, when the threshold value of the transistors varies due to manufacturing variations or the ambient temperature, voltage detection cannot be performed accurately. Further, in a flip-flop circuit, power supply voltage (VDD) is supplied via a capacitor (C2). It is therefore assumed that the power supply voltage rises at a fixed rate or greater.
The present invention provides a power on detection circuit that accurately detects the rising of an input with a simple structure and reduced current consumption.